The invention relates to magneto-optical structures designated to application in systems for optical processing of information, and can be utilized for building sensors, converters of magnetic field, and other devices of similar purpose.
At the present time, ferrite-garnet materials containing bismuth in form of thin or multi-layer films (foils) are widely known, and are used for visualization and observation of magnetic fields.
In particular, epitaxial films containing bismuth are used for observation of magnetic fields from various sources, as well as for processing of information in magneto-optical devices and systems [1,2]. Such epitaxial films are structures grown on mono-crystalline underlay of gadolinium gallium garnet (GGG) having their crystallographic orientation in the usual plane [111], [110] or [210].
Well known are magneto-optical films with garnet structure of (Bi,Y,Tm,Gd).sub.3 (Fe,Ga).sub.5 O.sub.12, applicable to display devices, optical devices for data processing, and storage components [3]. As presented on FIG. 1, such film 1, epitaxially grown on a GGG underlay 2 with [111] orientation, has a mono-axial anisotropy directed along orientation line [111]. The faraday rotation coefficient of such a film, measured with a going-through 546.1 nanometer wavelength light, is of the order of 3 deg/micrometer. Such a film exhibits a B-H hysteresis of the Faraday rotation as a function of the field applied along the [111] axis. The field magnitude, required for switching the film between opposite saturation states, varies in the range of 30-400 E.
A film, similar to the one presented by FIG. 1, has mono-axial direction of magnetization, i.e. vector 3 of the magnetization is oriented perpendicular to the film plane. Characteristic of such films is the "labyrinth" domain structure, schematically presented on FIG. 1. It forms in absence of external magnetic field H.sub.ext, perpendicular to the film plane, or when such field is weak.
When external field H.sub.ext of sufficient level is applied to film 1, its domain structure changes, approximately representing the form of the magnetic flux coming from the corresponding source. This is the base principle of visualization of magnetic field using ferrite-garnet films containing bismuth. Inclusion of bismuth in the composition of the ferrite-garnet film improves its magneto-optical properties. The relatively low resolution of such films, the limited resolution caused by the width of stripe domains, can be considered a disadvantage. The width of stripe domains can be reduced by increasing the saturation magnetization 4PM.sub.s of the film. This, however, results in strong reduction of sensitivity of such film to the external magnetic field H.sub.ext. The saturation magnetization 4PM.sub.s can also be increased by reducing the thickness of the film. Such approach, however, leads to the reduction of Faraday rotation of polarized light vector making observation of the visualized magnetic field difficult. Resolution achieved with mono-axial ferrite-garnet films containing bismuth is not better than 1.2 micrometers for magnetic field sources having signal/noise ratio in the range of 45-50 dB.
Also known are magneto-optical structures with bismuth containing ferrite-garnet films, in which magnetization vector M is oriented along the film plane, so called films with "easy plane" [4,5]. Such a structure is schematically presented on FIG. 2. Usually a GGG mono-crystal or GGG with complex substitution by Ca Mg Zr, or any other mono-crystalline dielectric material with similar crystal parameters of crystal lattice serves as underlay 2 for such a film. Orientation of the underlay for ferrite-garnet films with "easy plane" of magnetization can be [111], [210], or [100]. Under influence of perpendicular component of the external magnetic field H.sub.1ext the vector 3 of magnetization M deflects from the film plane at certain angle which depends on the magnitude of the external field H.sub.1ext. The angle of Faraday rotation of light polarization vector is proportional to the angle between M and the film plane, and proportional to the H.sub.1ext.
The main advantages of films with "easy plane" are high resolution and capability to achieve deep optical modulation, i.e. to obtain higher contrast image of the magnetic field. This capability is bound to the higher values of the Faraday rotation. However, in order to achieve such values it is necessary to introduce higher number of bismuth ions into the crystallographic lattice of the film. On the other hand, increasing the number of bismuth ions increases anisotropy of the field H.sub.a of the ferrite-garnet film and, in result, raises the value of the H.sub.1ext field necessary for obtaining the desired angle of vector M rotation. In the other words, sensitivity of the film goes down. Furthermore, in result of introducing bismuth ions in the film composition above certain number, the film becomes mono-axial.